'Kosuke Heki, a geophysicist at Hokkaido University in Sapporo, Japan, first got interested in the subject when he spotted an increase in the total electron content of the ionosphere—the charged outermost layer of the atmosphere—above Tohoku about 40 minutes before the magnitude 9.0 earthquake struck in 2011. Heki had long used GPS data to study ionospheric responses to earthquakes, which occur when the sudden movement of Earth’s crust reverberates through the atmosphere. Ionospheric disturbances interfere with the communication between GPS satellites and receivers, leaving a fingerprint at specific radio frequencies that researchers can tease out.

'In 2011, Heki was skeptical of electromagnetic precursors. But since then, he has used the world’s growing array of GPS stations to identify similar signals before nine other major earthquakes, he explained at the meeting. In addition, Heki has found that earlier anomalies precede stronger earthquakes, potentially reflecting the longer time needed to initiate rupture along larger segments of a fault. Now, he says he’s convinced there’s really something going on: “Seeing is believing.”

'However, scientists have yet to agree on a mechanism by which the crust could create electromagnetic signals. One idea is that rocks can generate positive charges when heated or stressed in the build-up to an earthquake, says Friedemann Freund, an adjunct professor of physics at San Jose State University in California and a senior scientist at NASA’s Ames Research Center in Mountain View, California. “When you stress a rock, it turns into a battery,” Freund says. “Not an electrochemical battery that you find in your car, but a new type of semiconductor battery that produces electrons and holes.”'